a. Field of the Invention
This invention relates to a method and apparatus for forming a stable, satisfactory image on an image recording medium through the electrophotographic technique.
b. Description of the Prior Arts
As an electrophotographic technique, the image forming process defined in terms of the Carlson process, etc., has been widely known, in which an electrostatic latent image is formed on a photosensitive body made of an electrically conductive layer and a photoconductive layer in a laminar form. Also, another method and apparatus for forming such an electrostatic latent image, in which such image is formed by the use of a photosensitive screen having a multitude of tiny openings, have been disclosed in Japanese Patent Publication No. 50-18782 (U.S. Pat. No. 3,647,271) and a Laid-Open Japanese Patent Application No. 51-341 (U.S. Ser. No. 771,309).
With progress in the above-described electrophotographic technique, there have been developed process control techniques for stabilizing the obtained image. As an example, there is a method in which electric current caused by a corona discharger to charge the photosensitive body is detected, and the thus detected current is controlled to a constant level to thereby stabilize the electrostatic latent image in an indirect manner. In the case of using the photosensitive screen, there is such a method that an electrostatic latent image formed by modulation is rendered constant (Laid-Open Japanese Patent Application No. 51-1322). In these methods, it is important for obtaining the images in their optimum conditions to properly effect the detection and control of the electric current in conjunction with the actual image forming process cycle, although none of the disclosures that point.
For the modulating element which modulates ion current and charged toner particles, there have been known Japanese Patent Publications No. 47-33775 and No. 49-20094, in which an electrical signal is applied to an electrode in the form of an aperture board, etc.. These publications do not also mention anything about such important factors as appropriate detection and control of ion current and charged toner particles. In the case of using the photosensitive screen system, there is such a method that multiple modulations of the corona ion current is made possible with one and same primary electrostatic latent image formed on the screen, whereby multiple numbers of reproduction copies can be obtained. In the image forming technique using such screen, there inevitably takes place changes in the image quality with increase in the number of the modulation times. In order therefore to correct such changes and to form the reproduced images substantially free from such changes, a corrective device to vary the corona ion current for the secondary electrostatic latent image formation becomes necessary. For such corrective means, there can be contemplated a method, in which various conditions for the image formation is caused to vary in accordance with preset conditions based on the number of times for the ion current modulation. It is, however, practically difficult to correct the image changes to a sufficiently adequate state under the constant preset conditions due to various factors such as difference in the construction of the screen accompanied by its fabrication conditions, changes in the state of the screen owing to its state of use, and changes in the environment of the screen, and others. Further, when the ion current is modulated for multiple numbers of times by the use of the primary electrostatic latent image, there inevitably arises a limitation. It can therefore be contemplated that this number of times for the modulation is preset to be the limitation, and that a subsequent primary electrostatic latent image is reformed at a stage when this preset number of times for the modulation is reached. However, since the number of times for the modulation varies depending on the screen conditions, the number of the modulation times to be preset is slightly less than that practically possible. On account of this, there would occur such a situation that the modulation has to be terminated on the way, in spite of the fact that such modulation is still possible practically, and the primary electrostatic latent image should be reformed with the consequent decrease in the image forming speed.
In the electrophotographic technique, the electrostatic latent image which has been formed is developed with a developing agent, or developer. For the developer, there is used color particles called "toner". This toner is, in most cases, used in the form of a mixture with a carrier such as glass beads, iron powder, organic solvent, and so forth. In this developing system, since the mixing ratio between the toner and the carrier (hereinafter simply called "toner concentration") greatly affects the image quality, the control of the toner concentration constitutes one of the important techniques in the electrophotographic reproduction method. For the automatic control method of the toner concentration, there have been known the following: (1) a method, in which the toner concentration is measured, depending on excess or shortage of which a toner feeding mechanism is operated; and (2) a method, in which a quantity of the toner to be actually consumed is measured or estimated, and, depending on the actual consumption thereof, the toner is replenished. In the former method, however, there has so far been no satisfactory measuring expedient and the result of the measurement, when done, is lack in fidelity. For the latter, there has been known such a method that average density of the original image is measured, or another latent image separate from that for the image reproduction is formed, and then this separate latent image is developed to replenish an amount of the toner used for the development. In reality, however, this method is remote from its realization in consideration of the fatigue in the measuring means for the average density of the original image, and in the toner due to the development.